Enhancement: Add `--ncdu` option to `ls` command

NCDU (NCurses Disk Usage) is a tool to analyse disk usage of directories.
It has an option to save a directory tree and analyse it later.
The `ls` command now supports the `--ncdu` option which outputs information
about a snapshot in the NCDU format.

You can use it as follows: `restic ls latest --ncdu | ncdu -f -`

https://github.com/restic/restic/issues/4549
https://github.com/restic/restic/pull/4550
